Uploaded by amohammadturza


Slides are taken from Berkeley EECS Department
•What is Nachos?
– Capabilities, purpose, history
•How does it work?
•How do I get started?
What is Nachos?
• An instructional operating system
• Includes many facets of a real OS:
Virtual Memory
I/O driven by interrupts
• You can (and will) modify and extend it
What else is Nachos?
• Nachos also contains some hardware
– MIPS processor
• Can handle MIPS code in standard COFF,
except for floating point instructions
• You can (and will) write code in C, compile it to
MIPS and run it on Nachos.
– Console
– Network interface
– Timer
Why Nachos?
• What better way to learn how an OS
works than by building one?
• Much easier and more reasonable to
build a simulated one in Java
• Skeleton code allows us to work on,
replace, or upgrade one piece at a time.
History of Nachos
• Originally created here at Berkeley in
1992 in C++
• By Wayne A. Christopher, Steven J.
Procter, and Thomas E. Anderson
• Used at many universities
• Rewritten in Java by Daniel Hettena
– Now simpler, easier to grade, type-safe,
portable, and more students now know
How are we using it?
• Four Nachos assignments - “Phases”
• Phase 1 - Threading
• Phase 2 - Multiprogramming
• Phase 3 - Caching and Virtual Memory
• Phase 4 - Networks and Distributed
How does Nachos work?
• Entirely written in Java
• Broken into Java packages:
nachos.ag (autograder classes)
nachos.machine (most of the action)
nachos.network (Phase 4)
nachos.security (tracks priviledge)
nachos.threads (Phase 1)
nachos.userprog (Phase 2)
nachos.vm (Phase 3)
Booting Nachos
• When you run Nachos, it starts in
• Machine.main initializes devices interrupt controller, timer, MIPS
processor, console, file system
• Passes control to the autograder.
• AutoGrader will create a kernel and
start it (this starts the OS)
The Machine!
• nachos.machine.Machine
• Kicks off the system, and provides
access to various hardware devices:
Interrupt Controller
• Kicks off hardware interrupts
• nachos.machine.Interrupt class
maintains an event queue, clock
• Clock ticks under two conditions:
– One tick for executing a MIPS instruction
– Ten ticks for re-enabling interrupts
• After any tick, Interrupt checks for
pending interrupts, and runs them.
• Calls device event handler
Interrupt Controller (cont.)
• Important methods, accessible to other
hardware simulation devices:
schedule() takes a time, handler
tick() takes a boolean (1 or 10 ticks)
checkIfDue() invokes due interrupts
• All hardware devices depend on
interrupts - they don’t get threads.
• nachos.machine.Timer
• Hardware device causes interrupts
about every 500 ticks (not exact)
• Important methods:
– getTime() tells many ticks so far
– setInterruptHandler() tells the timer what
to do when it goes off
• Provides preemption
Serial Console
• Java interface nachos.machine.SerialConsole
• Contains methods:
– readByte() returns one byte (or -1) and waits to
interrupt when it has more
– writeByte() takes one byte and waits to interrupt
when its ready for more
– setInterruptHandlers() tells the console who to call
when it receives data or finishes sending data
• Normally implemented by
nachos.machine.StandardConsole, hooked up
to stdin and stdout
Other Hardware Devices
• Disk
– Didn’t make the jump to Java from C++, we
don’t use it for our Nachos assignments
• Network Link
– Similar to console, but packet based.
– Used for Phase 4.
– You should be able to figure it out by then.
The Kernel
• Abstract class nachos.machine.Kernel
• Important methods
initialize() initializes the kernel, duh!
selfTest() performs test (not used by ag)
run() runs any user code (none for 1st phase)
terminate() Game over. Never returns.
• Each Phase will have its own Kernel
Oh, how I hated the kernel, with his wee beady eyes, and
smug look on his face! “Oh, you’re gonna buy my chicken!”
• Happens in package nachos.threads
• All Nachos threads are instances of
nachos.thread.KThread (or subclass)
• KThread has status
– New, Ready, Running, Blocked, Finished
• Every KThread also has a
• Internally implemented by Java threads
Running threads
• Create a java.lang.Runnable(), make a
Kthread, and call fork().
• Example:
class Sprinter implements Runnable {
public void run() {
// run real fast
Sprinter s = new Sprinter();
new KThread(s).fork();
• Some subclass of
• Creates ThreadQueue objects which
decide what thread to run next.
• Defaults to RoundRobinScheduler
• Specified in Nachos configuration file
Nachos Configuration
• nachos.conf file lets you specify many
– which clases to use for Kernel, Scheduler
– whether to be able to run user progs
– etc.
• Different one for each project.
• One step at a time. Get a little bit
working. Then a little more. Then a
little more, etc.
• Find a good tool, including a debugger,
and use it, like Eclipse and NetBeans.